1. Field of the Invention
The present disclosure relates to battery management systems.
2. Description of Related Art
Electronic machines, such as mobile phone terminals, digital cameras, tablet terminals, portable music players, handheld game consoles and laptops, are powered by various types of batteries; these machines are configured with a rechargeable battery (secondary battery), whereas the central processing unit (CPU) for system control and signal processing, liquid crystal panel, wireless communication module, and other electronic circuits like analog or digital circuits are powered by the electric energy from the battery.
FIG. 1 is a block diagram of a battery-driven electronic machine. The electronic machine 500 comprises a battery 502 and a charging circuit 504 for charging the battery 502. The charging circuit 504 receives the power supply voltage VADP from an external power adaptor or universal serial bus (USB) to charge the battery 502.
The battery 502 connects with a load 508. The current IBAT flowing through the battery 502 becomes the difference of the current ICHG from the charging circuit 504 and the load current (discharging current) ILOAD flowing through the load 508.
In the battery-driven electronic machine, the detection of the remaining capacity (state of charge, SOC) of the battery has become an indispensable function; hence, a remaining capacity detection circuit 506 is disposed in the electronic machine 500. The remaining capacity detection circuit 506 is also referred to as the Whewell gauge integrated circuit (IC). The voltage method and coulomb calculation method (charge accumulation method) are two mainstream methods for detecting the remaining capacity of the battery using the remaining capacity detection circuit 506. There are also some cases in which the charging circuit 504 is placed in the remaining capacity detection circuit 506.
In the voltage method, the open circuit voltage OCV of the battery is determined in an open state (no-load state), and the remaining capacity is determined according to the corresponding relationship between the OCV and SOC. It is not feasible to determine OCV when the battery is in the no-load and non-relaxed state, and therefore, it is not possible to correctly determine the remaining capacity during the charging or discharging.
In the coulomb calculation method, the remaining capacity is determined by integrating the charging current flowing into the battery and the discharging current flowing out from the battery (hereinafter referred to as charge and discharge current) to calculate the amount of charged electric charges and the amount of discharged electric charges with respect to the battery. According to the coulomb calculation method, during the operating period of the battery in which an open voltage cannot be obtained, it is also feasible to determine the remaining capacity; this is a difference between the coulomb calculation method and the voltage method.
In FIG. 1, the remaining capacity detection circuit 506 uses the coulomb calculation method to determine the remaining capacity of the battery 502. The remaining capacity detection circuit 506 comprises a coulomb counter circuit 510 and an SOC calculation unit 512. The coulomb counter circuit 510 detects the current IBAT of the battery 502 and integrates the same. The coulomb count value CC generated by the coulomb counter circuit 510 is expressed as the following equation.CC=∫IBATdt
Strictly speaking, the current IBAT of the battery is sampled by discrete-time sampling, which is calculated according to the following equation, wherein Δt represents the sampling period.CC=Σ(Δt×IBAT)
The summation (integration) is carried out, for example, by setting the current IBAT flowing out from the battery 502 as positive and the current IBAT flowing into the battery 502 as negative.
The SOC calculation unit 512 calculates the SOC of the battery 502 based on the coulomb count value CC. The calculation of the SOC is according to the following equation.SOC [%]=(CCFULL−CC)/CCFULL×100CCFULL represents the amount of electric charges (coulomb count value) stored in the battery 502 when it is fully charged.